The answer is actually relatively simple for such a confusing time. The Democrat party had split at the Baltimore convention, leading most Southern Delegates nominating Breckenridge for President and most Northern delegates to nominating Douglas for President. Lincoln was nominated on the Republican ticket on the third round, and enjoyed almost exclusively Northern support (although with substantial support in Missouri). John Bell's Union party was primarily former Whigs(who had broken up by 1856) who were ideologically opposed to the Democrats, many of them shared Republican economic ideology but the Republican anti-slave platform made it difficult for most former Whigs to support Lincoln so they backed Bell and attempted to exclude the issue of slavery entirely. Certainly the split among the Democrats was the biggest factor in so many viable candidates, an issue I can speak more to.